Electrical cables for the high speed transmission of electrical signals are well known. One common type of electrical cable is a coaxial cable. High speed transmission cables generally include an electrically conductive central conductor(s) or wire(s) surrounded by an insulating dielectric layer. An exemplary high speed transmission cable is a coaxial cable. In a coaxial cable, the electrically conductive conductor and insulating dielectric layer can further include an outer conductor and a protective outer jacket.
The insulating dielectric layer can be composed of any material or combination of materials that electrically separate the central conductor from other conductors within the cable. The material properties of the dielectric layer can significantly affect the transmission of the electrical signal along the length of a high speed transmission cable. Minimal interaction between the electric field and the dielectric layer is generally desired to maintain the signal integrity and to reduce the capacitance of the electrical signal. Capacitance slows the propagation rate of the electrical signal and reduces the signal strength. Additionally, capacitance is a strong contributor to the cable's impedance, and therefore the dielectric layer has the role of influencing the magnitude and uniformity of the cable impedance, which is generally desired to be a constant along the length of a given insulated wire. Key electrical properties influenced by the material properties of the dielectric layer include signal attenuation, signal propagation rate, capacitance per given cable length, impedance, and the uniformity of these electrical properties along the length of the cable. Conversely, it may be desirable for the cable to have prescribed electrical properties, such as a known impedance value. Prescribing these electrical properties will impact the structure and dimensions of the dielectric layer. The dielectric structure and the material's dielectric constant will directly influence the required thickness of the dielectric layer and hence the cable diameter, the cable flexibility, and related properties.
For example, the velocity of propagation (VOP) of electrical signal along a coax cable relative to the speed of the electrical signal along a conductor surrounded by air is:
      V    ⁢    O    ⁢    P    =      1                  ɛ        eff            
where εeff is the effective dielectric constant of the dielectric layer surrounding the central conductor. The dielectric constant of air is virtually equal to one while solid dielectric materials have a dielectric constant of greater than one. In order to maximize the velocity of propagation of the electrical signal, the effective dielectric constant of the dielectric layer should be minimized. The inclusion of air into the dielectric layer is one way to reduce the effective dielectric constant of the dielectric layer.
Although electrical properties of the transmission cable generally improve with the incorporation of air into the dielectric structure, air alone (at ambient pressure) can not provide adequate support to counteract external forces that can be applied to the cable during manufacture, installation and use of the cable. Failure to support the external load at any point can result in local distortions of the spacing between the central conductor and surrounding structures of the cable, thereby changing the distribution of the electric and magnetic fields around the central conductor, creating local impedance changes which can result in signal reflections and degraded signal integrity. If these distortions are significantly large (like a kink in the cable) or numerous, the cable may no longer be suitable as a high speed transmission line. Because air alone is not a sufficient support, the dielectric layer will also include a higher stiffness material form and maintain the space between the inner conductor and the surrounding structures of the cable.
Three types of dielectric layer structures which include a significant amount of air surrounding the central conductor are routinely practiced in the art: A) foamed and expanded polymers, B) thin helically wound monofilaments and, C) axially-extruded channels.
Foamed or expanded structures can have air content up to about 70% resulting in an effective dielectric constant to 1.3-1.5. However, the stiffness of the resulting dielectric layer can be quite low, and may fail to provide sufficient support to the central conductor under applied loads and may allow the central conductor to kink when tightly bent. When loaded, these structures readily buckle and crush.
The helically-wound structures typically utilize a monofilament or deviations thereof that are wrapped around a central conductor. An insulator tube is extruded over the wrapped conductor structure. These helically-wound structures can also have low effective dielectric constants (−1.3), but typically provide support against external forces at one point around the circumference of the central conductor at any given cross-section. This individual contact point can also be insufficient to support external load exerted at any point around the circumference of the central conductor that is not directly adjacent to the wrapped filament which can lead to local deformations or kinking of the central conductor on bending and result in attendant signal integrity issues.
The third type of dielectric layer structures which include a significant amount of air are longitudinally extruded structures formed along the conductor axis with a modified extrusion tip. These extruded structures can generally result in an effective dielectric constant of 1.45 or higher, but the axial extrusion process of a molten polymer is not well-suited to providing small, closely-spaced features since surface tension and the dynamics of extruding a liquid material in this manner drives rounding of the features. Additionally, this process cannot readily form features that vary along the axial direction, (i.e. each cross section profile is the same). Also, the process is limited to materials that can be extruded around a conductor at the required thickness.
In summary, the prior art dielectric structures do not have sufficient ability to provide low effective dielectric constants combined with sufficient mechanical integrity and design flexibility. A need exists for high speed transmission cables that include a dielectric layer that incorporates a significant amount of air adjacent to and around the central conductor while providing more uniform support around the central conductor resulting in a dielectric layer having greater mechanical stability while simultaneously having a low effective dielectric constant.